Highly purified fibronectin matrix has been isolated from cultured fibroblasts. The new procedures lead to the matrix with only one protein component, i.e., fibronectin, free from actin, myosin or other protein's contaminants. This matrix will be used to study: (1)\the associated glycosaminoglycans, if any; (2)\the basis for fibronectin being in a polymerized state; (3)\binding properties; and (4)\effects on the differentiation of adipocytes, myocardiac cells, granulocytes, macrophages, HL-60 promyelocytic cells, erythroid Friend's cells as well as MDCK kidney cells dorm formation, capillary endothelium formation and more complete differentiation of fibroblasts. Relevancy of the in vitro study to the role of fibronectin in vivo will be investigated. Reconstitution of tissues with purified fibronectin matrix will be studied. Mechanisms of fibronectin matrix-induced morphological reversion of transformed fibroblasts will be characterized as well.